Silicone rubber compositions have been used for a variety of applications in the past. Silicone rubber is desirable because it is waterproof, it is generally unaffected by wide temperature variations, and it resists the deteriorating effects of acids, bases, salts and ultraviolet (UV) radiation. Furthermore, silicone rubber is flame retardant and thus is suitable for applications requiring increased protection from fire. The flame retardant nature of the coating can be enhanced by the addition of nonflammable aggregates to the coating, for example, sand.
One primary application for silicone rubber has been as a coating for existing building roofs. Traditional roofing elements such as shingles can be made from a variety of materials including tile, slate, wood, concrete, and compositions of asphalt and aggregate. All of such materials experience weathering to varying degrees, with the relatively inexpensive asphalt composition shingles and commercial roofing typically being the most affected by prolonged exposure to UV radiation, temperature variations, and other environmental conditions. Wood shingles are generally more durable than composition shingles and are more aesthetically pleasing. However, because of the flammability of wood shingles, they have been banned in many areas as a fire hazard. Clay tile and slate roofs present little, if any, fire risk but are so expensive that they have not experienced widespread usage. Concrete shingles are durable and fire resistant but are heavy in comparison to other types of materials. Concrete shingles also readily absorb water because of their porous nature, further adding to the load which must be supported by the structural elements of the roof. The disadvantages of each of these materials are lessened by the addition of a layer of silicone rubber. By coating with silicone rubber, wood shingles become flame retardant, concrete shingles become water resistant, and the remaining roofing materials become significantly more resistant to the most common causes of deterioration, thereby lengthening the useful life of the shingles. Furthermore, silicone rubber coatings are not slick to walk on but instead provide a less hazardous surface having sufficient friction to help minimize slipping.
Application of a coating of silicone rubber to other surfaces besides rigid shingle materials can also be extremely advantageous. One example of using silicone rubber to protect a surface is disclosed in U.S. Pat. No. 4,297,265, which discloses the application of a solubilized silicone rubber and silicon dioxide composition onto flexible substrates such as glass cloth for use as awnings or other flexible building structures.
The flexibility, light weight, and durability of silicone rubber provides a number of suitable applications. Other uses of silicone rubber are also disclosed in U.S. Pat. Nos. 2,751,314, 2,934,464, 2,979,420, and 3,455,762. Furthermore, U.S. Pat. No. 5,338,783 discloses some silicone rubber compositions suitable for the above-described applications, such compositions comprising a mixture of silicone rubber, silicon dioxide, and an aggregate such as sand, gravel, and cinders.
While uses for silicone rubber are known, the physical application potential of silicone rubber has not been realized because the application of silicone rubber in many applications before the present invention was not possible or impracticable. Because of the difficulty of applying silicone rubber, the full benefits of this composition have not been realized. Silicone rubber, generally in the form of a silicone-aggregate mixture, possesses several characteristics which make it extremely difficult to apply in an efficient economical manner, such as a spray, while maintaining an appropriate coating consistency.
The first problem with applying a silicone-aggregate mixture occurs because it has a very high viscosity. This high viscosity requires a high pressure pump in order to force movement of the mixture through the apparatus. Furthermore, the required pressure usually increases as the distance the mixture is transported to the point of application increases. This is particularly so in roofing situations where it is highly desirable that the mixture be mixed at a location more stable than the roof and then pumped up to the elevation of the roof. The inventors have found that the high pressure required makes rubber hoses impractical. Rubber swelled in diameter until the hose failed.
A problem related to the high viscosity of silicone-aggregate mixtures is the requirement that the mixture be flowable enough to be pumped through the apparatus and through the hose and spray nozzle. The previous methods do not include an operational spraying apparatus for the various problems specific to silicone-aggregate mixtures, but instead anticipates applying a mixture manually. As a result of this manual application, the prior formulations have not been concerned with obtaining a mixture flowable enough to pass through the spraying apparatus, yet not so flowable that it runs and smears and results in a layer insufficiently thick on the application surface. Accordingly, associated with the development of a spraying apparatus is the need to develop a mixture within a desirable range of characteristics.
An additional problem occurs if the silicone-aggregate mixture is exposed prematurely to too much moisture. When this happens, cross linking occurs and the mixture begins to set up and eventually sets up and becomes solid. If cross linking of the mixture goes too far before it is discharged from the apparatus, then blockage of the apparatus occurs and it eventually fails completely. This was an additional problem faced by the inventors when rubber hoses were tested to transport the mixture. Air and moisture penetrated into the hose because of the porosity of the rubber. At a certain level of moisture in the hose, the mixture cross linked sufficiently to block the hose, preventing transport of the mixture.
Furthermore, silicone adheres to most materials, thus, it was discovered that even when the required pressure could be attained, the mixture would attach itself to the internal components of the spraying apparatus and quickly block the flow. Of the few materials to which silicone does not adhere (including polytetrafluoroethylene, polyethylene, and polypropylene), it was not known in the art to use such a material such that it would withstand the requirements of spraying a silicone-aggregate mixture, such requirements including the increased pressure and the necessity of preventing moisture from entering the mixture.
An additional problem in development of the invention was cleaning the apparatus following use. With the spraying apparatus in general, it is possible to simply flush the material out of the machine and the hose directly following use. However, with silicone-aggregate mixtures the material remaining inside the machine and the hose simply smears because of its high viscosity and its adhesion properties. The mixture which adhered to the walls would react with the moisture and air and cure the silicone. As a result, reacted silicone built up in the apparatus and hoses after cleaning. This would cause partial or complete blockages after repeated use. Furthermore, options such as replacing the hose every couple of uses is not economically feasible because of the expense involved in obtaining new equipment.
Similar problems exist with various other high viscosity polymer and high aggregate mixtures. Accordingly, the apparatus of the present invention is also suitable for other types of mixtures that present comparable application difficulties. One example of another suitable mixture is an acrylic coating mixture.
As a result of the shortcomings of the art, a need exists for a mixing and spraying apparatus which can accommodate the extreme requirements associated with applying silicone-aggregate mixtures and other high viscosity mixtures in the form of a spray. Specifically, the mixing and spraying apparatus must be powerful enough to overcome the high viscosity of such a mixture, must properly isolate the mixture from atmospheric air and other sources of excess moisture, must be formed of materials that minimize the amount of the mixture adhering to the apparatus, and must be easily cleanable and reusable.
Accordingly, it is an object of this invention to provide an apparatus and method for mixing and spraying a silicone-aggregate mixture onto surfaces which are not readily accessible for manual application or have surfaces that make uniform application difficult. Such surfaces can include roofs, awnings (and other flexible building structures), ship hulls, and any other surfaces which would benefit from a uniform layer of a waterproof, flame retardant, and lightweight material that is resistant to deterioration caused by acids, bases, salts, and ultraviolet (UV) radiation.
It is also an object of this invention to provide an apparatus and method for mixing and spraying a high viscosity mixture which can produce and withstand the elevated pressure necessary to force such a mixture through the apparatus and out of the spray nozzle.
It is another object of this invention to provide an apparatus and method for mixing and spraying a high viscosity mixture, particularly a silicone-aggregate mixture, that has a flowability characteristic within a viscosity range of about 5,000 to about 60,000 cps.
It is a further object of this invention to provide an apparatus and method for mixing and spraying a silicone-aggregate mixture in which excess moisture from atmospheric air and other sources will not come into contact with the silicone-aggregate mixture, thereby preventing cross linking of the mixture and subsequent blockage of the apparatus.
It is a still further object of this invention to provide an apparatus and method for mixing and spraying a silicone-aggregate mixture in which the hose used for transporting the mixture from the mixing apparatus to the point of application is lined internally with a material to which silicone does not adhere, for example, polytetrafluoroethylene (Teflon), polyethylene, or polypropylene.
It is yet another object of this invention to provide an apparatus and method for mixing and spraying a high viscosity mixture such as silicone-aggregate which is cleanable and reusable.
It is a further object of this invention to provide an apparatus and method for mixing and spraying a high viscosity mixture which repeatedly mixes the mixture prior to application to prevent excessive viscosity, and which filters out clumps of unmixed or solidified materials above a certain maximum size.
It is also an object of this invention to provide an apparatus and method for mixing and spraying a high viscosity mixture such as silicone-aggregate wherein the mixture is sprayed in a substantially uniform pattern with a substantially uniform thickness.
The present invention has many advantages which include: (a) metering mechanisms to maintain the proper ratio of solid and liquid components; (b) a filter screen before the pump to provide the operator with a visual indication of consistency of the mixture so that adjustments may be made; (c) the filter screen also minimizes the entry of overly viscous mixture into the pump; (d) a receiving chamber and an agitator that process the mixture before it reaches the pump so that the spraying operation may be intermittently started and stopped; (e) a hose with a coating of polytetrafluoroethylene (Teflon) and sealable ends that allow for unreacted mixture to be stored in the hose between usage, thereby minimizing waste from cleanup and man hours for cleanup; (f) a pump with a replaceable stator for ease of maintenance; and (g) a sealable pump apparatus to allow storage of unreacted mixture in the pump between usage.
Additional objects and advantages will become apparent and a more thorough and comprehensive understanding may be had from the following description taken in conjunction with the accompanying drawings forming a part of this specification.